Methods of growing heteroepitaxial single crystal or large grained semiconductor films and devices thereon

ABSTRACT

A method is provided for making smooth crystalline semiconductor thin-films and hole and electron transport films for solar cells and other electronic devices. Such semiconductor films have an average roughness of 3.4 nm thus allowing for effective deposition of additional semiconductor film layers such as perovskites for tandem solar cell structures which require extremely smooth surfaces for high quality device fabrication.

The present invention is a Continuation-in-Part of U.S. patentapplication Ser. No. 14/979,285, filed Dec. 22, 2015, which is aContinuation-in-Part of U.S. patent application Ser. No. 14/724,066,filed May 28, 2015, which is a Continuation-in-Part of U.S. patentapplication Ser. No. 14/224,675 filed Mar. 25, 2014, now abandoned,which is a continuation of U.S. patent application Ser. No. 13/929,085filed Jun. 27, 2013, now U.S. Pat. No. 9,722,130, which is acontinuation of U.S. patent application Ser. No. 12/903,750 filed Oct.13, 2010, now U.S. Pat. No. 8,491,718, which is a continuation-in-partof U.S. patent application Ser. No. 12/774,465 filed May 5, 2010, nowU.S. Pat. No. 9,054,249, which is a continuation of U.S. patentapplication Ser. No. 12/154,802 filed May 28, 2008, now abandoned, allof which are hereby incorporated by reference in their entirety.

REFERENCES CITED

U.S. Patent Documents

-   U.S. Pat. No. 4,717,688 January 1987 Jaentsch . . . 148/171-   U.S. Pat. No. 5,326,719 July 1994 Green et al. . . . 427/74-   U.S. Pat. No. 5,544,616 August 1996 Ciszek et al. . . . 117/60-   U.S. Pat. No. 6,429,035 August 2002 Nakagawa et al. . . . 438/57-   U.S. Pat. No. 6,784,139 August 2004 Sankar et al. . . . 505/230

OTHER PUBLICATIONS

-   Kass et al, Liquid Phase Epitaxy of Silicon: Potentialities and    Prospects”, Physica B, Vol. 129, 161 (1985).-   Massalski et al, “Binary Alloy Phase Diagrams”, 2^(nd) edition,    (1990), ASM International.-   Findikoglu et al, “Well-oriented Silicon Thin Films with High    Carrier Mobility on Polycrystalline Substrates”, Adv. Materials,    Vol. 17, 1527, (2005).-   Teplin et al, “A Proposed Route to Thin Film Crystal Si Using    Biaxially Textured Foreign Template Layers” Conference paper    NREL/CP-520-38977, November 2005.-   Goyal et al., “The RABiTS approach: Using Rolling-assisted Biaxially    Textured Substrates for High-performance YBCO Superconductors,” MRS    Bulletin, Vol. 29, 552, (2004).-   Nast et al, “Aluminum Induced Crystallization of Amorphous Silicon    on Glass Substrates Above and Below the Eutectic Temperature”, Appl.    Phys. Lett., Vol 73, 3214, (1998).-   Girault et al, “Liquid Phase Epitaxy of Silicon at very low    Temperatures”, J. Crystal Growth, Vol. 37, 169 (1977).-   Kayes et al, “Comparison of the Device Physics Principles of Planar    and Radial p-n junction Nanorod Solar Cells”, J. Appl. Phys., Vol.    97, 114302, (2005).

FIELD OF THE INVENTION

The present invention is related to producing large grained to singlecrystal semiconductor films, such as silicon films, for producingarticles such as photovoltaic and other electronic devices.

FEDERAL FUNDING

None

BACKGROUND OF THE INVENTION

It is widely known that radiation from the sun striking earth providesenough energy to supply all of mankind's needs for energy for theindefinite future. Such a source of energy can be clean andenvironmentally benign.

It is also widely known that global warming is associated with the useof fossil fuels, such as coal, oil, and natural gas. It is accepted bythe scientific community that global warming can have severe adverseeffects around the planet. There are numerous efforts around the world,combined with a sense of urgency, to cut down emissions from the usageof fossil fuels. A dominant factor in favor of the continual use offossil fuels is their cost per unit of available energy. If, forexample, the cost of producing photovoltaic cells can be reduced by afactor of approximately three while maintaining efficiency ofconversion, the photovoltaic technology would become cost competitivewith fossil fuels.

A major cost component in photovoltaic cells is the cost of thesubstrate on which the semiconductor film capable of converting sunlightinto electricity is placed. The most widely used substrate is singlecrystal silicon (Si). These substrates developed for themicroelectronics industry have been modified for application inphotovoltaic technology. If a silicon film could be deposited on aninexpensive substrate, such as glass, and with comparable quality asthat found in silicon single crystals used in the microelectronicsindustry, the cost of photovoltaic technology would drop significantly.

Epitaxial growth of thin films is a very well established process. Ithas been investigated by hundreds of researchers. Epitaxial depositionprovides a very viable way of growing very good quality films. Manysingle crystal semiconductors and insulator surfaces are used to studythe epitaxial growth of metallic films; for example, the growth ofsilver on silicon, sapphire, or a mica surface. Epitaxial metallic filmshave also been grown on other metallic films, such as gold on silver. Incontrast to metals, semiconductors, such as silicon, are difficult togrow epitaxially. For example, heteroepitaxial films of silicon havebeen successfully grown only on sapphire but at temperatures that arerelatively high for the applications we disclose here, such as thegrowth of silicon on glass substrates.

In order to take advantage of highly textured large grained films forphotovoltaic technology two problems need to be solved: inexpensivegrowth of high quality films and the availability of an inexpensivesubstrate on which desirable properties can be achieved. Here, wedisclose a method for growing semiconductor films, such as silicon,satisfying the two requirements listed above and suitable forphotovoltaic technology and other electronic applications.

The thermodynamic stability and formation temperature of two or moreelements is described by a composition versus temperature diagram,called a phase diagram. In this invention we shall make use of phasediagrams. These phase diagrams are available in the scientificliterature (Massalski et al). The phase diagram provides information onthe behavior of different phases, solid or liquid as a function oftemperature and composition. For example, the liquidus in a simplebinary eutectic system, such as Au and Si, shows how the relativecomposition of the liquid and solid, it is in equilibrium with, changeswith temperature. It is therefore possible to choose an averagecomposition, different from the eutectic composition, and cool themixture in such a way as to precipitate out one phase or the other. Ifthe composition is chosen to be richer in silicon than the eutecticcomposition then on cooling through the liquidus boundary between thesingle phase liquid and the two phase liquid plus solid, silicon willnucleate and form a solid phase. If on the other hand it is gold richrelative to the eutectic composition the first solid phase to nucleateis gold rather than silicon.

At and below the eutectic temperature the two components, in this case,Au and Si solidify from the liquid phase to phase separate into the twocomponents Au and Si. The interface energy between the two components isgenerally positive and therefore drives the two components to aggregateinto distinct phases with a minimum of surface area between the tworather than a fine mixture of the two. There is, however, the energeticsof two other interfaces to consider also: one with the substrate and theother with vacuum or gas. In considering energetics it is not only thechemical interaction of the metal or Si with the substrate that isimportant but also its crystallographic orientation, for the surface orinterface energy depends upon orientation of the grains. Another concernis the difference in lattice match between the nucleating film and thesubstrate which can lead to strain induced energy that is minimized byeither inducing defects or not growing uniformly in thickness across thesubstrate surface. These factors determine if silicon is likely todeposit on the substrate (heterogeneous nucleation) or nucleate andforms small crystals in the liquid (homogeneous nucleation).

An advantage of using eutectics compositions is that the eutectictemperature is lower than the melting temperature of the constituentelements. For example, the eutectic temperatures of Au, Al, and Ag withSi are 363, 577, and 835 degrees Centigrade (° C.), respectively. Incontrast the melting temperatures of the elements are 1064, 660, and961° C., respectively. The melting temperature of silicon is 1414° C.The eutectics then offer the possibility of nucleating a silicon crystalfrom the liquid far below the temperature at which pure liquid siliconcrystallizes. By a proper choice of the substrate surface exposed to thenucleating silicon, it is possible to nucleate and grow single crystalor large grained silicon films.

We have discussed silicon eutectics using elements such as Au, Ag, andAl. However, it is possible to replace the elements by silicon basedcompounds. For example, the compound nickel silicide forms a eutecticwith Si. There are numerous other examples of silicide compounds forminga eutectic with Si (Massalski et al). An advantage of using a silicideis that frequently the electrical contact of the silicide with siliconhas very desirable properties, such as a good ohmic contact or aSchottky barrier. Some silicides are also known to have an epitaxialrelationship with silicon. In this case, by appropriately choosingeither a silicide rich or silicon rich melt either the silicon can beinduced to grow epitaxially on the silicide or the silicide on silicon.A disadvantage in this approach is the eutectic temperature, which isgenerally high.

Low temperature solutions can also be formed with some elements, Forexample, gallium (Ga) and Si have a eutectic temperature of less than30° C., very close to that of the melting point of Ga. There are otherelements, such as indium or tin that form low temperature liquidsolutions with silicon. Si can be nucleated from these solutions at verylow temperatures relative to pure silicon (Girault et al, Kass et al).These temperatures are sufficiently low that it opens up the possibilityof using organic materials as substrates on which large grained tosingle crystal films can be grown. While this is an advantage, there isalso a serious disadvantage; at these low temperatures, the silicon filmcan contain defects and hence are not very useful as a photovoltaicmaterial. However, these very low temperature deposits can be used toinitiate the nucleation of a very thin silicon film, which issubsequently thickened by using higher temperature processes to optimizeits photovoltaic properties.

The choice of a particular system (phase diagram) is not only determinedby temperature and energetics of the interfaces, but also by thesolubility of the second element in Si. It is desirable to have precisecontrol of the doping of Si in order to optimize its semiconductorproperties for photovoltaic applications. It is also important to selectthe composition of the substrate and temperature of processing such thatthere is minimal or no chemical interaction between the silicon film andthe surface of the substrate on which it is being deposited. From thepreceding description, we can extract five common points which arerelevant to this invention. First, one end of the phase diagram alwayshas the semiconductor we wish to nucleate and use to produce a film, wehave used silicon in the preceding examples but it could be germanium(or any other semiconductor material from Group IV) or a compound suchas gallium arsenide (or any other semiconductor material from GroupIII-V) or cadmium selenide (or any other semiconductor material fromGroup II-VI). Second, the thermodynamically predicted concentration ofthe second element or phase in the semiconductor is minimal. If there issolubility then it must be a desirable dopant. For example aluminum (Al)in silicon behaves as a p-type dopant and experience in thesemiconductor industry has shown that trace amount of Al can bedesirable. Third, the liquidus curve has the highest temperature on thesemiconductor side. In other words, the melting point of thesemiconductor is greater than the liquidus for all compositions inequilibrium with the semiconductor. Fourth, the homogeneous nucleationenergy of silicon crystal from the melt is greater than that forheterogeneous nucleation on the substrate. This latter conditionpromotes heterogeneous nucleation. And, fifth, the temperature forepitaxial growth is low enough to use inexpensive substrates such asglass but high enough to promote a good quality silicon film. Forexample, a growth temperature above approximately 550 degrees Centigrade(550° C.) is desirable to make a good quality silicon film. Thesoftening temperature of ordinary glasses is around 600° C. Thesoftening temperature of borosilicate glasses is higher. However it isnot high enough to use conventional deposition temperature of greaterthan 750 degrees Centigrade for silicon on insulator, such as a sapphiresubstrate.

In order to take full advantage of the invention disclosed here thesemiconductor material has to be deposited on a substrate material whichis inexpensive, and the surface of which enables heterogeneousnucleation and growth. In the following we shall discuss two specificmethods for producing substrates suitable for heterogeneous depositionof films for photovoltaic technology. Both of these methods have beendescribed in the scientific literature and we do not claim to inventthem. We include them here for completeness.

The use of rolled and textured Ni and Ni-alloy sheets has been proposedas substrate material for superconducting films and, more recently, forfilms for photovoltaic devices (Findikoglu et al). In order tofacilitate the growth of epitaxial superconducting films on suchsubstrates, there have been two approaches described in the scientificliterature: in one the sharp rolling texture produced in a rolled andannealed Ni alloy is used as a template on which various epitaxialbuffer layers are deposited followed finally by an epitaxial film of ahigh temperature cuprate superconductors (Goyal et al). In the secondapproach (Findikoglu et al), the nickel ribbon is used as a substratefor ion beam assisted deposition of a wide variety of highly texturedceramics, for example, magnesium oxide (MgO). The ion beam aligns thegrowing MgO film, which provides a template for the subsequentdeposition of the cuprate superconductor.

The latter approach is not limited to using metal tapes but can beextended to other inexpensive substrates such as glass (Teplin et al).It has been found that texture can also be induced in MgO by depositingthe film on a substrate that is inclined to the normal from the oncomingvapor of MgO.

One limitation of the use of glass as a substrate has been its softeningtemperature, which is generally lower than the conventional processingtemperatures required for the growth of large grained or single crystalfilms of silicon. With the method of depositing silicon films at lowtemperatures, described in this invention, the use of buffered glassbecomes an option for we can deposit highly textured and large grainedsilicon on MgO at or below the softening temperature of glass.Similarly, researchers have grown crystalline aluminum oxide (Al₂O₃) oninexpensive substrates (Findikoglu et al). We shall use MgO and Al₂O₃ asillustrative examples. However, it is understood to those skilled in theart that a variety of other materials can also work. Both Findikoglu etal and Goyal et al describe other buffer layers, including conductingceramic layers, such as TiN.

OBJECT OF THE INVENTION

It is an object of the present invention to provide single crystal orhighly textured relatively large grained good quality semiconductorfilms and, in particular silicon films, for photovoltaic technology orother semiconductor devices, such as field effect transistors used, forexample, in displays.

It is yet another object of this invention to provide single crystal orhighly textured relatively large grained good quality semiconductorfilms and, in particular silicon films, at low temperatures. Forexample, if silicon films are used, the growth temperature is between450 and 750 degrees Centigrade.

It is yet another object of this invention to provide single crystal orhighly textured relatively large grained good quality semiconductorfilms and, in particular silicon films, on inexpensive substrates, forexample, substrates such as glass on which buffer layers such as MgOand/or Al₂O₃ have been deposited.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the forgoing andother objects can be achieved by alloying a semiconductor and, inparticular silicon, with elements or compounds that form an eutecticsystem, and increasing slowly the concentration of the semiconductor,such as silicon, through the liquidus line to reach the two phase regionin which the semiconductor, in particular silicon, nucleates out of themelt and on the surface of a substrate.

In accordance with another aspect of the present invention, the forgoingand other objects can be achieved by alloying a semiconductor and, inparticular silicon, with elements or compounds that form an eutecticsystem, and increasing slowly the concentration of the semiconductor,such as silicon, through the liquidus line to reach the two phase regionin which the semiconductor, in particular silicon, nucleates on thesurface of a substrate to produce a highly textured relatively largegrained or single crystalline film.

In accordance with yet another aspect of the present invention, theforgoing and other objects can be achieved by alloying a semiconductorand, in particular silicon, with elements or compounds that form aneutectic system, and increasing slowly the concentration of thesemiconductor, such as silicon, through the liquidus line to reach thetwo phase region in which the semiconductor, in particular silicon,nucleates on the surface of a substrate made of a buffered tape in whichtexture is produced by mechanical deformation and the buffer layers areepitaxial to the texture of the metal tape. The buffer layer exposed tothe melt comprises of compounds, such as Al₂O₃ or MgO.

In accordance with yet another aspect of the present invention, theforgoing and other objects can be achieved by alloying a semiconductorand, in particular silicon, with elements or compounds that form aneutectic system, and increasing slowly the concentration of thesemiconductor, such as silicon, through the liquidus line to reach thetwo phase region in which the semiconductor, in particular silicon,nucleates on the surface of a substrate made of a buffered tape, a glasssubstrate, or any other material suitable for inexpensive manufacture ofphotovoltaic cells in which strong texture is produced by ion beamassisted deposition. The final layer, which is exposed to the siliconmelt, comprises of compounds, such as Al₂O₃ or MgO.

In accordance with still another aspect of the present invention, theforgoing and other objects can be achieved by using a solid phasecomposition comprising a semiconductor and, in particular silicon, withelements or compounds that form an eutectic system, and in which a thinfilm of the element or compound is deposited first followed by thesemiconductor, such as silicon, and depositing at a temperature wherethe semiconductor atoms diffuse through the element or compound toheterogeneously nucleate on the substrate and propagate thiscrystallinity to the semiconductor film remaining on top of the elementor compound.

The method of manufacture of materials suitable for photovoltaictechnologies described in this invention are much less expensive in theconversion of sunlight into electricity than those practiced in theprior art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the phase diagram of the eutectic system Au—Si, taken fromthe literature (Massalski et al). The melting points of the two elementsAu and Si, as well as the eutectic temperature are shown in the figure.The eutectic composition is also indicated. The liquidus line, whichdefines the boundary between the liquid gold-silicon alloy and solidsilicon and a gold-silicon liquid alloy, and on the silicon rich side ofthe phase diagram, is marked. The figure also shows the change in phasesas the composition is changed by depositing silicon on a film of goldheld at constant temperature. As the silicon is evaporated on to thegold film, the film comprises of gold solid and a liquid gold-siliconalloy which changes from the point marked by 11 towards 12. Furtherdeposition of silicon results in the film entering the liquid phaseregion between the points marked 12 and 13. As the silicon depositioncontinues beyond the point 13, the liquidus boundary, solid siliconnucleates from the liquid which is in equilibrium with a silicon-goldliquid alloy. The solid silicon is deposited on a MgO substrate, forminga highly textured and relatively large grained heterogeneously nucleatedfilm. The thickness of the solid silicon film increases till thedeposition is stopped. As it cools Si continues to deposit from the meltwhile the Au—Si liquid solution becomes richer in gold.

This process continues till the eutectic temperature is reached, atwhich point the liquid solidifies and phase separates into gold andsilicon solids.

We have used the phase diagram of the Au—Si eutectic. The Al—Si eutecticis very similar. Here we can heterogeneously nucleate silicon from theAl—Si melt on a single crystal sapphire substrate to form a singlecrystal heteroepitaxial silicon film.

FIG. 2 shows an image of a scanning electron microscopy of silicon thinfilms deposited on flexible glass for a sample with 40 nm Al/200 nm Si.

FIG. 3 shows an image of a scanning electron microscopy of silicon thinfilms deposited on flexible glass for a sample with 100 nm Al/200 nm Si.

FIG. 4 shows an EDS (Energy-dispersive X-ray Spectroscopy) spectrum ofone data point of a silicon film grown using the method disclosedherein. The figure represents a very low amount of Au-contaminationwithin the semiconductor film layer.

FIG. 5 shows the minor amount of the gold (Au) remnants on top of asilicon film grown using the method of the present invention, afteretching.

FIG. 6 shows a STEM/HAADF image taken of a semiconductor silicon filmproduced according to the method described in Example 6, discussed indetail below. The image shows some Au (gold) metal located on top of thesilicon layer but no Au (gold) metal within the silicon layer orlattice.

FIG. 7 shows a roughness characterization of the Si—Au crystallinethin-film according to Example 7.

FIG. 8 shows the results of an optical profilometer of Si—Au crystallinethin-film according to Example 7 having an average roughness of 3.4 nm.

FIG. 9 shows a XRD image showing only one silicon peak.

DETAILED DESCRIPTION OF THE INVENTION

As described above, we have disclosed a method to produce low costsingle crystal or large grained epitaxially aligned good qualitysemiconductor films, in particular silicon, for photovoltaic technology.We have also suggested the use of tapes or glass slabs as substratematerials. The tapes provide strong texture on which buffer layerssuitable for silicon growth are present. Our method can produce siliconepitaxy at substantially lower temperatures than those commonlypracticed, hence not only minimizing interaction with the surface of thesubstrate but also enabling the use of glass substrates.

We shall be using the eutectics of silicon with gold and aluminum indescribing the details of the invention. It is, however, understood thatone skilled in the art can extend the methodology to othersemiconductors such as germanium, gallium arsenide, or the cadmiumselenide class of photovoltaic materials. Furthermore, other metals mayalso be used in a similar eutectic relationship and composition withsilicon. Such metals include but are not limited to Sn (tin), Ag(silver), Cu (copper), In (Indium), Ni (nickel), and Ga (gallium).

FIG. 1 shows the phase diagram of the eutectic system Au—Si. Theeutectic composition is nominally 18.6 atomic percent pct Si and therest being gold. A thin gold film is first deposited on the bufferedsubstrate. This is followed by silicon deposition. As the siliconconcentration increases the film first forms a two phase mixture of goldand liquid gold-silicon. The composition of the latter is determined bythe choice of the deposition temperature. With further increase ofsilicon, the liquid phase region, marked 12, is reached and theremaining gold is dissolved. With still further increase of the amountof silicon, the second liquidus phase boundary, marked 13, is reachedand subsequent deposition of silicon atoms results in a solid phase ofsilicon in equilibrium with the silicon-gold liquid. If the substratesurface is suitably chosen, for example MgO crystals, the solid siliconnucleates heterogeneously onto the surface. The choice of thetemperature of deposition is determined by balancing two considerations:quality in terms of defects of the epitaxial film; too low a temperatureor too rapid a growth rate of the film at that temperature can introducedefects versus too high a temperature when chemical interaction ormechanical integrity of the substrate limit the usefulness of thematerial.

We have started with vapor deposition of the metallic film and addedsilicon to it to traverse the phase diagram from point marked 11 in thefigure. However, the metallic element and silicon can be co evaporatedto reach any concentration between the points marked 12 and 13 in thefigure and subsequently silicon added to reach the desired thickness,before cooling to room temperature.

When the desired thickness of the silicon film is obtained, thesubstrate with the film is cooled to room temperature. Even though theamount of gold required to catalyze a silicon film is small, it can befurther reduced by etching the gold away, for example, by using iodineetch, available commercially. This gold can be recycled.

In one embodiment of the present invention, the semiconductor materialis deposited on an inexpensive substrate material and the surface ofwhich enables heterogeneous nucleation and growth. In another embodimentof the invention, the semiconductor can be applied directly on glasswithout any buffer layer added. In this latter case, the same depositionprocess is used but the materials are deposited directly on glass. Forexample, a eutectic alloy composed of Al and Si are used to deposit Sion glass. The amount of Al and Si can be varied depending on the desiredoutcome, as can the temperature. It is noted that when depositingdirectly on flexible glass, the greater the Al thickness and higher thetemperature, the better the Si crystallinity will be. FIG. 2 shows SEM(scanning electron microscopy) of silicon thin films deposited onflexible glass as completed on a sample with 40 nm Al/200 nm Si. FIG. 2shows sporadic large Si crystallites (a few microns in size) across thesurface with granular continuity. The granular film was characterized assmaller Si crystallites mixed with some amorphous Si as confirmed byRaman spectroscopy.

Upon increasing the thickness of the aluminum layer, the large Sicrystallites appeared more continuous across the surface, while thesurrounding granular film morphed into a higher degree crystalline Si,as illustrated by SEM imaging. This trend continued through the thickestAl tested sample, with the granular film significantly retreating fromthe enlarged and connected Si crystallites and giving way to asurrounding crystalline Si dominated film, as illustrated in the SEM ofthe second sample with 100 nm Al/200 nm Si. See FIG. 3.

Therefore, the results demonstrate that the crystallization is dependenton the thickness ratio of Al over Si, with the increased Al contentassisting the growth of higher crystallinity of Si thin film.

EXAMPLES OF THE INVENTION

The following non-limiting examples are used as illustrations of thevarious aspects and features of this invention.

Example 1

A good high vacuum system with two electron beam guns, is used todeposit gold and silicon independently. A glass substrate coated withion beam assisted deposited MgO film is held at temperatures between 575and 600° C. These are nominal temperatures. It is understood to oneskilled in the art that lower or higher temperatures can also be useddepending upon the softening temperature of the glass substrate or thereaction kinetics of either gold or silicon with the metallic tape orits buffer layers when used as substrates. A thin gold film ofapproximately 10 nm thickness is deposited first. This is followed by asilicon film deposited at a rate of 2 nm per minute on top of the goldfilm. The ratio of the thickness of the gold and silicon films is chosensuch that the final composition ensures that a point, marked 13, in FIG.1 is reached. This point lies at the boundary between the two phaseregion of solid Si and a liquid Si—Au mixture. For example, for a 10 nmgold film followed a 100 nm silicon film satisfies this condition.Additional silicon film nucleates heterogeneously on the MgO surface toform the desired thin film. The film can now be cooled to roomtemperature, where the film now comprises of two phases: gold and arelatively large grained and highly textured film of silicon on MgO.

By relatively large grained it is understood to imply a grain sizelarger than would have been achieved if a silicon film had beendeposited under the same conditions but without Au. In the examplediscussed above the crystallographic texture is strongly [111]. Insteadof an insulating substrate such as MgO, it is possible to choose stableand electrically conducting nitrides, such as TiN.

The gold diffuses to the surface of the silicon film, driven by itslower surface energy relative to the silicon surface. The film is etchedin a solution, such as a commercially available iodine based chemical,which removes the gold from the two phases, gold and silicon, leavingbehind a silicon film.

This silicon film can now be used as the surface on which a thickersilicon film appropriately doped to form a p-n junction, suitable forapplications such as photovoltaics, can be deposited. Alternatively, thethin silicon film can be used for heteroepitaxial deposition of othersemiconductors, which might be more efficient converters of sunlight toelectricity.

We have used two electron beam guns as an illustrative example. It isunderstood to one skilled in the art that other methods such as a singlegun with multiple hearths, chemical vapor deposition, thermal heating,or sputtering can also be used.

Example 2

A good high vacuum system with two electron beam guns is used to depositaluminum and silicon independently. A glass substrate or a Ni basedsubstrate coated with a buffer layer of Al₂O₃ is held at temperaturesbetween 600 and 615 degree ° C. These are nominal temperatures. It isunderstood to one skilled in the art that lower or higher temperaturescan also be used depending upon the softening temperature of the glasssubstrate or the reaction kinetics of either aluminum or silicon withthe metallic tape or its buffer layers when used a substrates. Theeutectic Al—Si is used instead of the Au—Si example above. A thin Alfilm 6 nm thick is deposited on the Al₂O₃ followed by a 100 nm thicksilicon deposition, and as described in example 1, above, the two phaseregion comprising of solid silicon and a liquid Si—Al mixture isreached. The deposition is stopped and the sample is slowly cooled toroom temperature. Aluminum diffuses through the silicon film, driven byits lower surface energy relative to silicon. The silicon film isheteroepitaxially aligned by the Al₂O₃ surface. The aluminum film on thesurface can be etched chemically by well-known processes to leave behinda silicon film. The surface of this film can now be used for furthergrowth of epitaxial films either for photovoltaic devices or for fieldeffect transistors.

We note, as stated earlier, that silicon can be grown epitaxially onsapphire but at temperatures higher than 750° C. This is awell-established commercial process. However, in the absence ofaluminum, silicon deposition at, say, 600° C. produces a fine grainedfilm rather than a heteroepitaxial film, as described above.

Example 3

We describe in this example how different methods of deposition can becombined to take advantage of highly textured films as described inexample 1, above. The Si film produced from the deposition of example 1is etched to remove the Au and then placed back into the vacuum chamberand p⁺-Si is deposited on this film. This latter layer serves twopurposes: it provides a conducting layer for a photovoltaic device to besubsequently built on it and can be the starting point for a variety ofdifferently configured photovoltaic devices as, for example, a nanowirephotovoltaic device. Here a 2-3 nm thick gold film is deposited on thesilicon using an electron gun. This 2-3 nm thick gold film breaks upinto nanoparticles and is the starting point used by a number ofinvestigators to use chemical vapor deposition to grow nanowires and usethese nanowires for photovoltaic devices. The difference is that we showhow an inexpensive buffered glass can be used rather than a relativelyexpensive single crystal Si substrate.

A second possibility is to deposit a Au film of thickness 5 nm asislands on a MgO buffered glass substrate, using lithographic or othermeans known in the art. A heavily doped silicon (p⁺ or n⁻) film is nowdeposited on the surface followed by a p- or n-type silicon usingelectron beam deposition, as described in example 1. The thickness ofthe heavily doped film is in the micron range whereas the lightly dopedfilm is of the order of 100 nm. The deposition process is now changedand chemical vapor deposition is used for subsequent deposition ofsuitably doped films of silicon, practiced in the art to grow siliconnanowire photovoltaic devices. The heavier doped silicon film serves thepurpose of a conducting layer. Using gold islands has the advantage ofcontrolling the nanowires diameter and length in order to maximize theefficiency of the photovoltaic cell (Kayes et al). Instead of using theinsulating MgO buffer layer, a conducting material such as TiN can beused.

Example 4

We describe how different methods of deposition and temperature can becombined to take advantage of films grown as described in Examples 1 and2, above to produce desirable device structures. Instead of depositingthe Al and Si films described in Example 2 above, by two electron beamheated sources in a vacuum, we deposit the Si by decomposition of silaneusing a chemical vapor deposition chamber. This is a well knownindustrial process. As in Example 2, we deposit a 6 nm thin film of Alon to a sapphire substrate held at 600° C. We then introduce silane gasinto the chamber. At these temperatures, the silane decomposes to form aSi film which reacts with the AI to produce a eutectic solution and whenthis solution is saturated with Si (the equivalent of point marked 13 inFIG. 1 for an Au—Si alloy) the Si precipitates to heterogeneouslynucleate to form an epitaxial film on the surface of the sapphiresubstrate. This film is continuous and can be doped by adding borane orphosphene gases to silane to obtain p or n type semiconductor behavior.This film can now be used as a basis to construct thin film photovoltaiccells or, alternatively, grow nanowires of Si on top of it by simplylowering the temperature of the substrate below the eutectic temperatureof AI-Si (577° C.). For example, if the temperature of deposition is500° C., Si nanowires will grow on top of the Si film. The Al particlesthat precipitate out of the AI-Si solution once the temperature of thesubstrates is below the eutectic temperature now catalytically reducethe silane gas to form nanowires, as described in the literature. Thesenanowires can be used to build electronic devices, includingphotovoltaic cells.

Example 5

The same process can be performed as in the above examples, but directlyon glass rather than on a buffered substrate.

A textured silicon seed layer on a glass substrate was grown from anAl—Si eutectic melt layer at 725° C. via e-beam evaporation technique. Asmooth, thin, and flexible glass with a relatively elevated glasstransition temperature was used as the substrate material for thisinvestigation. Prior to film deposition, the glass substrates werecleaned by the standard RCA procedure and dipped in a 2% HF-solution for30 seconds (GORKA). Aluminum pellets (99.999% in purity, Kurt J. Lesker)and silicon pellets (99.999% in purity, Kurt J. Lesker) were used as thethermal evaporator and e-beam evaporator material sources, respectively.Using a thermal evaporator, Al layers varying from 30 to 100 nm inthickness were deposited onto glass substrates under moderate vacuumconditions of 10-5 Torr. The sample was removed from the thermalevaporator, placed into the electron beam evaporator and taken to basevacuum levels of 10-7 Torr. A Si film of 200 nm thick was subsequentlydeposited onto the Al base layer while the substrate was held at 725° C.The deposition rate of Si was held constant at 6 nm/min. After thedeposition was completed, the samples were allowed to naturally cooldown to room temperature. No post-deposition film annealing wasrequired. XRD spectra revealed that a crystalline Si film hassuccessfully been grown on the glass substrate with no buffer layer orpost-processing annealing step and given that there is a relativelyintense Si [111] peak, the film can be characterized as textured.Further work would include optimization of the growth parameters towardsthe achievement of complete continuity of a crystalline silicon seedlayer atop the glass substrate followed by deposition of a thickersemiconductor film for devices of various kinds.

Example 6

The following example describes an exemplary method for producingcontamination-free semiconductor (“contamination-free,” as used herein,is defined as no observable metal in the crystal lattice of the siliconfilm, as shown via figures of STEM imaging and EDS herein) and metallayers, such that a semiconductor film produced has very few, if any,metal impurities therein. The metal film similarly has very fewsemiconductor impurities after the process exemplified is complete. Sucha contamination-free semiconductor film is entirely free of metalimpurities, or at least substantially free of metal impurities such thatit greatly lowers, and in some cases removes entirely, the potential ofthe semiconductor film to induce highly active recombination defectsnear mid-gap due to such metal impurities located within the film.Contamination-free semiconductor film is a further improvement upondefect-free semiconductor film because there are no impurities withinthe semiconductor film, no impurities within the metal film, and norecombination defects.

The following is an example of the method for producingcontamination-free semiconductor films using silicon and gold as thesemiconductor and metal, respectively. However, other semiconductormaterials such as germanium and other metals such as tin can also beused.

A good high vacuum system with two electron beam guns was used todeposit gold and silicon independently and directly on a single crystalMgO [111] wafer substrate. The substrate was held at a temperaturebetween 575° C. and 600° C. during deposition. A thin gold film ofapproximately 10 nm was deposited on the substrate first. This wasfollowed by a silicon film deposited at a rate of 2 nm per minute on topof the gold film. The film was then cooled to room temperature, allowingthe phases of the film to separate into gold and a crystalline film ofsilicon on the glass. Importantly, there was no annealing step.

Impurities in silicon influence solar-cell properties in a variety ofways. For example, crystal growth can be perturbed resulting in defects,inclusions, precipitates, or polycrystalline structure, thus causingsubstantial detrimental effects during application and employment of thecrystal. The bulk properties of the silicon may be altered byelectrically active impurity centers which reduce the minority carrierdiffusion length either by increased combination near midgap or byscattering-induced mobility loss. Additionally, impurities may inducecontact and contact interface degradation, series and shunt resistanceeffects, as well as precipitation and other junction defect mechanisms.(Davis et al “Impurities in Silicon Solar Cells”, IEEE 1980). Cellefficiency, particularly in high efficiency devices, is stronglydegraded by metal contaminants which reduce bulk diffusion length. Forexample, as few as 10¹¹ Ti atoms cm⁻³ can reduce the efficiency of an18.5% cell to 16.8%. (R. H. Hopkins et al “Impurity effects in Si forsolar cells”, Journal of Crystal Growth, 1986).

It is known that one atomic percent Au in a cubic centimeter (cm³) of Sicorresponds to 5×10²⁰ Au atoms. This is a very heavy doping level andcould be considered almost alloy. Typical doping levels insemiconductors range from 10¹⁶ to 10¹⁹; the higher level being adegenerate semiconductor. Deep level traps of 1% or less of the dopinglevel are not a problem for Si. But 1 atomic % is much higher. Thesilicon films grown here with Au have less than 0.1 atomic % based onthe results from performing Energy-dispersive X-ray spectroscopy (EDS)of the semiconductor film produced using the method of this example. Itshould be noted that similar films may be produced using Sn (tin), Ag(silver), Cu (copper), In (Indium), Ni (nickel), or Ga (gallium) in amanner similar to that which is described herein for Au (gold). Fourdata points were taken at 10 KV, and one data point at 7 KV. Bothresults show a concentration of Au (metal) below 0.1 atomic % within theSi (semiconductor) layer. Since this is a thin-film, spectra at 5different voltages on the sample were taken giving the composition andfilm thickness as well. FIG. 4 illustrates the EDS spectrum of just onedata point (7 KV) and shows a concentration of Au up to 1.6 atomic %,most likely due to scattering from Au remnants on top of the film afteretching, which can be seen in FIG. 5. The extremely low Au atomicpercentage in the Si film means that any leftover traces (below the 0.1atomic %) of Au in the film are so low that the Au cannot and will notinduce highly recombination active defects near mid-gap (i.e., the Audoes not cause substantial detrimental effects). It is noted that othersamples of the semiconductor film produced by the present method showleftover traces of Au (and other metals) as low as, and lower than, 0.01atomic percent, 0.001 atomic percent, 0.0001 atomic percent, 0.00001atomic percent, 0.000001 atomic percent, 0.0000001 atomic percent,0.00000001 atomic percent, and 0.000000001 atomic percent metal. It isgenerally understood that 5N or 6N is often the number cited for siliconpurity requirements for solar technology, where N represents the numberof 9's with regard to the purity of the silicon. For example, 5Ncorresponds to 99.999% pure silicon (and thus 0.0001 atomic percentmetal, or other contaminants or impurities). Therefore, at a minimum, asingle impurity must have a concentration less than 1 ppm, since thereare often many different kinds of impurities within the silicon.Additionally, they should all cumulatively be less than 0.0001 atomicpercent, if one wishes to remain in the 5N purity window. The integratedcircuits industry, in contrast, demands 9N pure silicon.

It is also noted that the extremely low Au atomic percentage in the Sifilm produced according to the present invention means that the Au issubstantially entirely recoverable (98 to 100% recoverable), meaningthat the cost of manufacture is not affected by using Au, thus allowingfor affordable scalability of the processes disclosed herein.

A STEM/HAADF (high angle annular dark field (HAADF) scanningtransmission electron microscopy (STEM)) imaging system was used toevaluate and validate the results of the film produced according to theabove-described method. It is generally known that STEM/HAADF offerssignificant benefits in dark field operation with a unique imaging mode,High Angle Annular Dark Field (HAADF) imaging. For example, the innerangle of the annular darkfield detector may be made so large (30milliradians) that no Bragg diffracted electrons are collected. Theimages therefore come from elastically scattered electrons which havepassed very close to the atomic nuclei in the sample. High (single atomcolumn) resolution is possible with no unwanted diffraction contrastwhich can mask structural information. The HAADF signal is directlyproportional to the density and thickness of the specimen andproportional to Z^(3/2) where Z is the atomic number. Thus it ispossible to produce images which show contrast due to the mass-thickness(i.e. where the signal is proportional to the number of atoms), or Zcontrast images (i.e. where the signal is proportional to the atomicnumber of the sample). HAADF is suitable for inorganic and organicsamples and for crystalline and amorphous materials. An additional bonusoffered by the STEM is the ability to collect secondary electrons andbackscattered images in the same way as a standard SEM. This makes itpossible to correlate surface information (from secondary electrons)with bulk information from the STEM mode. It is also possible to use thesecondary electron mode to image samples which are too thick even forSTEM observation. The high accelerating voltages available in the STEMoffer ultra-high resolution compared to a conventional SEM.

With regard to this particular example, STEM/HAADF images were taken ofthe semiconductor film prepared using the method of the presentinvention (see, for example, FIG. 6). These images show no signs ofmetal (e.g., Au) in the silicon film. Thus, the purity of the siliconfilm is at least 5N and such that any impurity, including but notlimited to Au, within the highly textured semiconductor film causes nosubstantial detrimental effect in the semiconductor silicon film.STEM/HAADF imaging is known to those of ordinary skill in the art to bea strong measure for determining Au (and other metal) impurity levels insemiconductor films, as discussed above. STEM images are directlyinterpretable, and the STEM lattice images would show Au as beinggreater than the Si, if Au were in fact located within the silicon layeror interface.

Example 7

The following example describes an exemplary method for making smoothcrystalline semiconductor thin-films and hole and electron transportfilms for solar cells and other electronic devices. Such semiconductorfilms have an average roughness of 3.4 nm thus allowing for effectivedeposition of additional semiconductor film layers such as perovskitesfor tandem solar cell structures which require extremely smooth surfacesfor high quality device fabrication.

Many electronic devices today are comprised of thin-films. Such filmsare typically deposited on a substrate such as glass, or a buffer layer,or even another semiconductor layer in the case of a tandem solar cellor a light emitting diode (LED). A monolithic tandem solar cell has twolayers, one deposited directly on another. For the cell to functionefficiently, the top layer should preferably be deposited on a smoothbottom layer. An example of such a cell is a perovskite/silicon thinfilm tandem. The main issues for practical device fabrication ofperovskite solar cells are film quality and thickness. Thelight-harvesting (active) perovskite layer needs to be several hundrednanometers thick—several times more than for standard organicphotovoltaics. Unless the deposition conditions and annealingtemperature are optimized, rough surfaces with incomplete coverage willform. Even with good optimization, there will still be a significantsurface roughness remaining. Therefore, thicker interface layers thanmight normally be used are also required. When depositing perovskitefilms (by spin coating, for example) on a bottom layer such as silicon,the bottom layer must be smooth or the perovskite film will be shuntedand therefore will have to be thicker in order to overcome the negativeeffect of the roughness of the surface of the bottom layer. An increasein thickness not only is more costly, but lowers the opticaltransmission, which in turn affects the overall device performancenegatively. A thin perovskite film is therefore preferable, but requiresa very smooth surface which is an involved task in the prior art. Asimpler, more effective method of fabricating smooth silicon thin filmsis therefore desirable.

In addition to the bottom layer being preferably smooth, the top layershould also be smooth because when solar cells or other devices arefabricated a conducting layer is deposited on the top layer for eitherhole transport or electron transport. Such ultrathin films called HTLsor ESLs require or benefit from a very smooth surface on which to bedeposited.

A simple and cost effective method for producing smooth thin films onsubstrates is disclosed in the previous examples, and described again inthe following example which is just one embodiment of a simple and costeffective method for producing smooth thin films on substrates. Thesesubstrates can include sapphire, buffered glass (MgO, Al2O3, TiN, andZrN, for example), and other inexpensive substrates. The example usessilicon and gold, but the process can be extended to other materialssuch as germanium, copper and tin, for example.

A good high vacuum system with two electron beam guns was used todeposit gold and silicon independently and directly on a single crystalMgO [111] wafer substrate. The deposition of the gold and silicon iscontinuous along the substrate. The substrate was held at a temperaturebetween 575° C. and 600° C. during deposition, the depositiontemperature range used for the thin gold film and silicon may be thesame. A thin gold film of approximately 10 nm was deposited on thesubstrate first. This was followed by a silicon film deposited at a rateof 2 nm per minute on top of the gold film. The film was then cooled toroom temperature, allowing the phases of the film to separate into goldwhich rises to the top and a smooth textured film of silicon contactingthe MgO wafer.

For device fabrication of solar cells, the gold can be etched, andeither an additional semiconductor film is deposited on top of thesilicon film for a tandem solar cell, or a hole transport layer (HTL) orelectron transport layer (ETL) is deposited on the silicon film andfollowed by a contact layer on top consisting of Ag or Au. Theadditional semiconductor film, HTL and ETL can all be deposited withsmoothness, replicating the layer it is deposited on since theunderlying silicon film is smooth.

In the present example silicon was used, but any inorganic film can beused, as can a variety of metals, such Sn, or Ni, which form a eutecticwith inorganic materials.

FIG. 7 shows the roughness characterization of the Si—Au crystallinethin-film sample on a MgO wafer of example 7. FIG. 8 shows the result ofan optical profilometer of the Si—Au crystalline thin-film according toExample 7 having an average roughness of 3.4 nm. Using an opticalprofilometer, an average roughness of the Si—Au film on the MgO [111]wafer substrate was measured at 3.4 nm which is very smooth andsufficient for perovskite film deposition, for example. A line-scanacross the sample showing heights in a single profile was alsoperformed. The scan area was quite large, about 250 μm². Also, the RMSdata shows double the average height, approximately 7.5 nm. FIG. 9 showsan XRD image which shows only one silicon peak of Example 7, indicatinga textured silicon film was grown on the MgO substrate.

In the present invention, the terms ‘textured’, ‘large grain’, and‘smooth’ are defined by the following definitions. The term ‘textured’means that the crystals in the film have preferential orientation eitherout-of-plane or in-plane or both. For example, in the present inventionthe films can be highly oriented out-of-plane, along the c-axis. Theterm ‘large grain’ is defined as a grain size larger than would havebeen achieved if a silicon (or other inorganic material) film had beendeposited under the same conditions but without metals, i.e. Au. Morespecifically, the term ‘large grain’ is defined as the grain size iscomparable to or larger than the carrier diffusion length such thatelectron-hole recombination at grain boundaries is negligible. Insemiconductor films this means that the grain size is greater than orequal to the film thickness. Finally, the term ‘smooth’ is defined as afilm, the semiconductor film for example, has an average roughness ofless than 10 nm, and an RMS of less than 20 nm.

The present invention greatly enhances device efficiency andeffectiveness.

While the principles of the invention have been described in connectionwith specific embodiments, it should be understood clearly that thedescriptions, along with the examples, are made by way of example andare not intended to limit the scope of this invention in any manner. Forexample, a variety of suitable substrates different from the examplesgiven above can be utilized or a different variety of deposition methodsand conditions can be employed as would be understood from thisinvention by one skilled in the art upon reading this document.

The invention claimed is:
 1. A method of growing a semiconductor film,comprising the steps of: providing a substrate; depositing a continuousmetal thin-film on said substrate; depositing a semiconductor film ontosaid continuous metal thin-film at a constant deposition temperature,said semiconductor film and said metal thin film forming a eutecticliquid, said constant deposition temperature being between a eutectictemperature of said eutectic liquid and below a softening temperature ofsaid substrate, and at said constant temperature, increasing aconcentration of said semiconductor such that the eutectic liquidbecomes saturated with said semiconductor and said semiconductornucleates from said eutectic liquid; cooling said eutectic liquid toroom temperature at which said eutectic liquid solidifies and said metalthin film and said semiconductor film separate, wherein said metal thinfilm rises above said semiconductor film and said semiconductor filmcontacts said substrate; and forming a textured semiconductor film onsaid substrate wherein said textured semiconductor film replicates atexture of said substrate, said textured semiconductor film having anaverage roughness of less than 10 nm, and RMS of less than 20 nm.
 2. Themethod of claim 1, wherein said semiconductor film is inorganic.
 3. Themethod of claim 1, wherein said metal is one of Sn, Al, Ag, Ni, Au andCu.
 4. The method of claim 1, further comprising: adding an additionalsemiconductor film, a hole transport layer or an electron transportlayer on top of said semiconductor film; and depositing a contact layeron top of said hole transport layer or said electron transport layer tofabricate a solar cell device.
 5. The method of claim 1, wherein saidaverage roughness is 3.4 nm, and the RMS is 7.5 nm.
 6. The method ofclaim 1, wherein an additional thin film is deposited on saidsemiconductor film, said additional thin film having an averageroughness of less than 10 nm and a RMS less than 20 nm.
 7. The method ofclaim 6, wherein said additional thin film is a semiconductor.
 8. Themethod of claim 6, wherein said additional thin film is a hole transportlayer (HTL).
 9. The method of claim 6, wherein said additional thin filmis an electron transport layer (ETL).
 10. The method of claim 1, whereinsaid method achieves a semiconductor film having a purity of 5N orgreater.